How does Rubashov’s experience with Arlova affect his appreciation of the “grammatical fiction”?
The "grammatical fiction" is what Rubashov experiences when his individuality defies Party bounds and appears, often when he has a toothache or is daydreaming. The grammatical fiction equates with the "silent partner" of his thought: his individual, unindoctrinated self.
While Rubashov was with the Trade Delegation, his secretary was Arlova. He and she had an affair during which she promised Rubashov that she will always be accessible to him. This is before she is warned to restock library books of political opposition leaders. When the Stalin purges begin, Rubashov stops joking with her and she stops going to his room. Soon she is condemned and killed as a traitor.
It is while daydreaming about Arlova on the seventh day of his imprisonment that has one of his experiences of grammatical fiction in which his individuality makes it difficult for him to follow thoughts to their logical conclusion in the Party way. After he is condemned and while he is awaiting execution, his grammatical fiction increases and he begins to ponder whether thee is purpose in life since he sees no paradise in the USSR.